The itemidglobal attribute is the unique, global identifier of an item. itemid attributes can only be specified on elements that have both itemscopeand itemtypeattributes. Also, the itemid can only be specified on elements with an itemscope attribute whose corresponding itemtype refers to or defines a vocabulary that supports global identifiers.

An itemtype's vocabulary will give the exact meaning of a global identifier defined within the vocabulary. The vocabulary will define whether several items with the same global identifier can co-exist and, if they are allowed, how items with the same identifier are handled.

Note: The Whatwg.org definition of an itemid says that it must be a URL. This does not seem to be the case as the example below shows. Rather the defacto definition is that an itemid is a unique identifier, so a URN is appropriate. This inconsistency no doubt reflects the incomplete nature of the Microdata specification.